Integrated Common And Differential Mode Filters With Active Damping For Active Front End Motor Drives

Abstract

IGBT based power converters acts as front end in the present day Adjustable Speed Drive (ASD). This offers many advantages and makes regenerative action possible. PWM rectifier operation produces electrically noisy DC bus on common mode basis. This results in higher ground current as compared to three phase diode bridge rectifier. Due to fast turn-ON and turn-OFF time of IGBT, the inverter output voltage dv/dt is high during switching transients and voltage waveform is rich in harmonics. As a result, in applications involving long cable the motor terminal voltage during the switching transient is as high as twice the applied voltage. This voltage stress reduces the life of insulation in motors. The high dv/dt output voltage applied at the motor terminal excites the parasitic capacitive coupling resulting in increased ground currents and causes Electric Discharge Machining (EDM) which reduces the life of motor bearings. The common mode voltage due to PWM rectifier and the inverter appear at the motor terminals exacerbating these problems. The common mode voltage due to PWM inverter with PWM rectifier is analyzed. An integrated approach for filter design is proposed wherein the adverse effects due to common mode voltage of both AFE converter and the inverter is addressed. The proposed topology addresses the problems of common mode voltage, common mode current and voltage doubling due to ASD. The design procedure for proposed filter topology is discussed with experimental results that validate the effectiveness of the filter. Inclusion of such higher order filter in the converter topology leads to problems such as resonance. Passive methods are investigated for damping the line resonance due to LCL filter and common mode resonance due to common mode filter. The need for active damping technique for resonance due to common mode filter is presented. State space based damping technique is proposed to effectively damp the resonance due to line filter and the common mode filter. Experimental results are presented that validate the effectiveness of active damping both on the line basis (differential mode) and line to ground basis (common mode) of the filter.